Doppler type correlation system



June 1i, 1968 D KRATZER ET Al. 3,388,398

DOPPLE-R TYPE CQRRELATION SYSTEM 4 Sheets-Sheet l Filed Oct. 24, 1966FFM `June l1, 1968 D, L KRATZER ET AL 3,388,398

DOPPLER TYPE CORRELATION SYSTEM Filed Oct. 24. 1966 4 Sheets-Sheet 2 "QMam;

#TT UEY June 1l, 1968 D, KRATZER ET AL 3,388,398

DOPPLER TYPE CORRELATION SYSTEM Filed Oct. 24. 1966 4 Sheets-Sheet 577425 47m/W727i fl ff//ff/ [f] 0 .f ,q [fj/[297) (j) afm- /wf/fff/xfwff)M) a we 4' JM l f/vag'fJ/@a/yefff @mi 0,132;

June 11, 1968 D. KRATZER ET AL 3,388,398

DOPPLER TYPE CORRELATION SYSTEM Filed Oct. 24. 1966 4 Sheets-Sheet 4Zena/4me W owahlqf;

United States Patent O 3,338,398 DGPELER TYPE CORRELATHN SYSTEM Dale L.Kratzer, Trainee, Pa., and Edward D. Cart'oiitc, Haddon Heights, NJ.,assigner-s to Radio Corporation of America, a corporation of DelawareFiled Get. 2d, 1%6, Ser. No. 583,778 d Claims. (Cl. 34E-17.5)

This invention relates to correlation systems, and particularly to radarsystems or the like that utilize the Doppler effect for providinginformation regarding moving objects.

rfhe invention will be described as embodied in a radar system designedparticularly for detecting and locating moving targets such as enemysoldiers or vehicles that are moving under cover of darkness or undercover of a forest, for example. ln this embodiment the radar system maybe operated either in an all range mode or in a range bins mode. Thepresent invention is concerned with the all range mode and thecombination of the two 1 modes. ln the all range mode, a moving targetanywhere within the maximum range of the radar set is quickly detected,assuming the antenna is pointed toward the target. After a target hasbeen detected by using the all range mode, the radar set is switched tothe range bins mode. By use of this mode the actual range of the targetis determined.

An object of the invention is to provide an improved Doppler typecorrelation system having an all range mode.

A further object of the invention is to provide a simplified system forthe all range mode of operation of a Doppler type correlation system.

A still further object of the invention is to provide an improvedDoppler type correlation system that may be operated to quickly locate atarget.

In the embodiment of the invention which will be described, the radarcarrier wave to be transmitted is phase modulated by a code whichrepeats at a comparatively high frequency. In the all range mode thiscode is a square wave. in the range bins mode this code is apseudo-random code. Each code changes in amplitude, the change beingfrom +1 to -l in the present example. When the code changes from +1 to-1 or from -1 to -l-l, the radar carrier is shifted 180 degrees inphase, ie., the phase of the carrier is reversed. With this type ofmodulation, the pseudo-random code may contain a comparatively smallnumber of bits (1023 bits in this example) so that it is practical todesign a system in which the pseudo-random code repeats at acomparatively high frequency (at a frequency of approximately 6000 persecond in this example). Thus, the reptition rate of the pseudorandomcode is readil, selected to be at least twice the frequency of thehighest frequency of the Doppler signal for which the operator listens.The square wave code for all range operation has a still higherrepetition rate when its quarter-wave period is selected to equal theradar transit time for a target at maximum range as will be discussedlater. In the present example the square wave code has a frequency of150,000 cycles per second. Because of these high repetition rates of thecodes it is comparatively easy, as will be discussed later, to filterout the ICC nonccrrelated signal from the Doppler signal heard by theradar operator. ln the present example, the highest Doppler frequencypassed to the operators earphones is 1500 cycles per second.

The square wave code for all range operation is generated very simply.Signal from a 6 megacycle clock that drives the pseudo-random codegenerator is applied to a frequency divider to obtain the 150,000 cycleper second square wave. The comparison signal that must be applied tothe correlator for comparison with the received radar signal isgenerated just as simply. As will be explained later, this comparisonsignal also is a square wave signal, but it has a frequency twice thatof the square wave code. In this example, the comparison signal for theall range mode is a square Wave recurring at 300,000 cycles per second.lt also is generated by a frequency divider fed from the 6 megacycleclock.

The invention will be described in detail with reference to theaccompanying drawing in which:

PEG. 1 is a block diagram of a radar system embodying the invention;

FIG. 2 is a circuit diagram of the receiver mixer of FIG. l and theassociated circuit for balancing out amplitude modulation from the phasemodulated carrier;

FIGS. 3 and 4 are groups of graphs that are referred to in explainingthe all range mode of operation of the system shown in FlG. 1; and

FIG. 5 is a diagram of a suitable phase modulator that may be used inthe system of FIG. 1.

ln the several figures like parts are indicatcdby similar referencecharacters.

Referring to FIG. 1, the radar carrier wave is generated by anoscillator 10 which supplies the carrier to a phase modulator 11. Thecarrier wave is designated cos LOG.

The oscillator 10 may supply a carrier having a frequency in the X band,or having a lower frequency if preferred. lf in the X band, the unit 10may be a stable oscillator that feeds into a non-linear device togenerate harmonics; a high frequency harmonic being sharply filtered topass a very clean X band carrier signal (a 9250 megacycle per secondsignal, for example) to the phase modulator 11.

At the phase modulator 11 the carrier is 180 degree phase modulated by acode supplied from a driver 12. This code is designated f(t). A specificexample of a suitable phase modulator will be described later. The phasemodulated carrier is supplied over a transmission line 13 to the firstport of a three-port circulator 14, and passes out of a second port overa transmission line 16 to an antenna 17 which preferably is directional.The phase modulated signal from the modulator 11, which is the signaltransmitted, may be written 1*(1) cos ont. Some of the signal from thephase modulator leaks through the circulator to the third port and feedsthrough a transmission line 18 and feeds with reected signal, asdiscussed below, into a receiver mixer 19.

Reflected signal from a target is picked up by the antenna 17. The roundtrip time of the radio wave transmitted to a target and reflected backto the radar set is designated f. Therefore, the reflected signal pickedup by the antenna is ,DHH-sr) cos wDU-l-f) where p is an attenuationconstant. The reflected signal feeds over the line 1d to the circulator14 where it adds to the leakage reference signal. This sum signal feedsfrom the third port through the line 18 into the mixer 19 where the sumsignal Ht) cos wt+pf(t|1) cos w(t+-r) is squared.

The transmission lines 13, 16 and 18, as well as the linesinterconnecting other units of the system, may be coaxial lines forexample.

The range bins mode of operation and the circuit for this operation willnow be described in more detail, to-

gether with the theory of operation which also applies to the all rangemode.

The pseudo-random code that is fed to phase modulator 11, it isgenerated by a pseudo-random code generator 21 known in the art. Thecode is illustrated schematically by the graph 9. It changes inamplitude from +1 to -.l. lf preferred, the code amplitude may be madeto change in -amplitude from +1 to O, for example. The code generatorillustrated comprises a l-bit or 10-stage shift register 22 that feedssignal from stage 8 and from stage l0 to a multiplier 23. The multiplieroutput feeds back to stage 1 of the shift register. The shift registeris driven by a clock 24 or master oscillator operating at 6 megacyclesper second. In this example, the code is 1023 bits long before itrepeats, there being one bit for each pulse from the clock. Therefore,the code repeats at a comparatively high frequency which is equal to theclock frequency divided by the number of bits in the code. Thus thepseudo-random code repetition frequency is per second approximately. Thepseudo-random code is taken off the first stage of the shift registerand supplied over lines 26 and 27, and through switch contact 28 andswitch arm 29 to the driver 12 when the switch arm 29 is in what isreferred to as the range bins position. The 'driver 12 may be anamplifier or any suitable circuit for increasing the amplitude of thecode suicient for operating the phase modulator 11.

The code is also taken off the shift register by way of one of the tentaps marked 1 to 10 and a tap switch 31. The taps 1 to 10 take signaloff shift register stages 1 to 10, respectively. At tap 1 the code isdelayed with respect to the undelayed code on line 26 by an amount equalto the transit time of the radar signal traveling to a target 25 metersfrom the radar and returning to the radar. The code appearing on tapswitch 31 is delayed an additional amount each time it is moved to ahigher number tap, the delay from one tap to the succeeding tap beingequal to the radar transit time to and return for a target 25 metersfrom the radar. Thus, at tap 10, for example, the code delay is equal tothe transit time of the radar signal going to and returning from atarget 250 meters from the radar set. The amount the code is delayed isdesignated T.

The delayed code f(t+T) from the tap switch 31 is supplied over a line32 to a balanced mixer or multiplier 33. The undelayed code f(t) issupplied over a line 34 to the multiplier 33 also. The unit 33 may beany suitable device or circuit that multiplies the two input signals togive the product signal f(t)f(t+T). This product signal appearing at theoutput of mixer 33 is supplied by way of a contact point 36 and a switcharm 37 over a line 35 to a correlator 38 as a comparison signal when theswitch arm is in the range bins position.

The output of the receiver mixer 19 is supplied through a videoamplifier 39 and a line 41 to the correlator 38. As will be expainedlater, the signal fed to the correlator by line 41 is the product of theundelayed code f(t), the transit time delayed code f(t+r), and acomponent cos wor representing Doppler signal due to target movement.That is, the signal on line 41 is f(t)f(t+f) cos w01-, This is the samesignal as the reference signal f(t)f(t+T) supplied to the correlatorover the line 35 when T equals r, except for the Doppler component.

The output of the correlator 38 is supplied to an audio amplifier 42which passes audio signal up to 1500 cycles per second in the presentexample. This is the Doppler signal cos wo-r which appears as thecorrelator output when the tap switch 31 is set on a shift register tapthat supplies code that is delayed by substantially the same amount -asthe received radar signal, that is, when the code delay T equals orsubstantially equals the radar signal delay r.

Cit

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The output of audio amplifier 42 is supplied to a utillization devicesuch as ear phones indicated at 43. In operating the radar, the operatorlistens to the ear phones, points `the antenna in the direction of asuspected moving target, and moves the tap switch 31 to the particulartap that results in the maximum amplitude signal in the ear phones,assuming the radar signal strikes a moving target so that a Dopplersignal is obtained. This particular tap gives the range of the target,the taps 1 to 10 representing ranges of 25 meters, 50 meters, etc., upto 250 meters, respectively. The direction of the target is indicated bythe direction in which the directional antenna is pointing.

The reason that the signal ;f(t)f(t+1) cos wor appears on the lead 41will now be explained. First, reference is made to FIG. 2 which shows asuitable balanced mixer circuit 19. The diode 19a is the one of presentinterest, the diode 19h being for balancing out any amplitude modulationthat may be present as will be described later. The diode 19a isprovided with a load resistor 51, the upper end of which is connected toa summing resistor S2 (summing for amplitude modulation cancellation). Atap S3 supplies the output of diode 19a (less amplitude modulation)through a coupling capacitor 54 to the video amplifier 39.

The sum signal or current e from the circulator is Kt) cos wt+pf(t+1)cos wo(t+v). This is squared by the diode 19a. The current output ofdiode 19a is e2=f2(t) cos2 w0t+2pf(t)f(t+1) cos wot cos YOU-FT)-l-PZZU-l-T) C032 woU-i-T) Since the trigonometric identity 1/2 (1+ cosZoot) may be substituted for cos2 wot and a substitution also may bemade using the trigonometric identity cos A cos B=1/2 cos (A+B)+l/2 cos(A-B), and since f(t) is either +1 or -1 whereby y2(t) is always +1 andf-(t+) is always -il, e2 may be written as follows:

The corresponding voltage V appearing at the tap 53 is a constant Ctimes e2. The voltage appearing at the output of the video amplifier 35is only the amplified cornponent Cpf(t)f(t+f) cos woe because the othercomponents of the squared signal are either direct current components orcomponents having a frequency twice that of the X band carrierfrequency, none of which is passed by the video amplifier. Thus, thesignal supplied over the line 41 to the correlator is f(t)f(t+1) cos w01(times a constant) as previously stated.

This video amplifier output signal on line 41 should be regarded ascontaining Doppler because rr, the round trip time. is not a fixedconstant number if the target is moving; in fact is a function of time,that is, the round trip time from the antenna to the target and backwill differ from time to time. Therefore, we may write r(t) which iscontained in the phase of the cosine term as a power series function,and expand it to Write 1-(1) as a constant term designated To (a fixedquantity) plus a linear term which may be written ict. Thus we mayexpress 1(1) as a constant time plus a linear function of time plushigher order terms as follows:

is the linear portion flot.

The reference signal applied over line 35 rto the correlator is`f()f(t|-T). Since a correlator multplies the two applied signals, itsoutput is When the tap switch 31 is set so that T=1, this output isf2(t)f2(r+fr`) cos wO-r. Since f2(t) and f2(t+r) are each equal to +1,this contracts to cos wor which is the Doppler frequency signal that isrepresentative of a moving target. When r is not equal to T, thefiltering or integration which is included in the correlator eliminatesthe broad spectrum signal.

Referring now in more detail to the all range mode, a target may belocated more quickly by first having the switch arm 29 set on a Contactpoint 46, and the switch arm 37 set on a contact point 47. This is ftheall range position of these switch arms for the all range mode ofoperation. T he contact point 46 is supplied from a frequency divider 48with a square wave voltage recurring at 150,000 cycles per second. Thedivider 48 is driven by the 6 megacycle clock 24.

The contact point 47 is supplied from a frequency divider 49 with asquare wave voltage recurring at 300,000 cycles per second. This is thecorrelator comparison signal for the all range mode. The divider 49 aisois driven by the clock 24.

By using the all range mode, the operator may quickly determine whetherthere is a moving target in the direction in which the antenna ispointing and anywhere within the maximum radar range, within 250 metersin this example. The operation of this mode will be explained withreference to the graphs in FIGS. 3 and 4.

Referring to FIG. 3, graph (a), this is the 150,000 cycles per secondsquare wave which is the code Ht) that phase modulates the carier wavein the all range mode operation. This is a 130 degree phase shiftmodulation just as in the range bins mode. This repetition rate orfrequency of the square wave is selected so that the time of one-fourthcycle of :the wave is equal to the transit time out and Vback for atarget at the maximum range, at 250 meters in this example. Thus, thereceived reflected signal from such a target is f(l-i1) and is delayedas indicated by FIG. 3, graph (b). As previously described, and asindicated in FIG. l, the output of the video amplifier 39 is f(t)f(t+r)times the Doppler signal cos wo-r. To simplify the graphs in FIGS. 3 and4, it is assumed that the targets are stationary so that there is noDoppler signal. In FIG. 3, graph (c) is the product of graphs (a) and(b), and thus represents the video amplifier output which is fed intothe correlator 38. This same signal should be fed into the correlatorover line 3S to obtain complete correlation for signal from a target `atmaximum range.

Such a signal, which is fed over line 35, is shown in FIG. 3, graph (f).It is the 300,000 cycles per second square wave supplied from frequencydivider 49. Referring to FIG. 3, graphs (d) and (e), it will be seenthat it corresponds to the product of the code f(t) and the delayed codef(t-{-T) where T=r, 1- in this case being the transit time where thetarget is at maximum range,

This all range mode provides a sensitivity time control (STC) effectbecause the correlation becomes less and less complete as lthe targetgets closer to the radar set. Maximum Doppler signal amplitude is heardwhen there is complete correlation.

This difference between complete and partial correlation is illustratedin a general way by the graphs of FIG. 4. Refer first to FIG. 4 wheregraphs (f), (g) and (lz) illustrate the complete correlation whichoccurs when the target is at the 250 meter maximum range. Graph (f)which represents the signal fed over line 41 into the correlator fromthe video amplifier. Graph (g) represents the 300,000 cycle signal fedover line 35 to the correlator, where the delay T equals the delay r.The product of these two signals (after multiplication by thecorrelator) is shown by graph (h). Since a stationary target has beenassumed, .this correlator output is direct current of a certain valueindicated as +1. The presence of Doppler signal would appear as avariation in the amplitude of this direct current.

Refer next to graphs (a), (b), (c), (d) and (e) of FIG. 4 whichillustrate the partial correlation that occurs for a target at 125meters from the radar set. Graph (c) is the product of the code f(t)shown in graph (a) and the received reflected code 7TH-r) shown in graph(b). This product signal is fed over the line 41 to the correlator.

The other signal fed to the correlator, by way of line 35, is the300,000 cycle wave shown by graph (d). The two signals, graphs (c) and(d), fed to'the correlator are multiplied in [the correlator to give theproduct signal shown by graph (e). After the filtering or integrationoccurring in the correlator, the correlator output is a direct currentof reduced value as compared with fthe value when there is the completecorrelation illustrated in graph (h). It will be evident that any directcurrent variation representing Doppler signal cannot be as great in thecase of partial correlation, graph (e), as in the case of compietecorrelation, graph (h), providing fthe larger direct current amplitude.

In the use of this all range mode, the operator will hear a Dopplersignal if there is a moving target anywhere within the 250 meter range.Therefore, in this mode the operator swings the antenna until he hears aDoppler signal. He then knows the antenna is pointing to a movingtarget. He then switches to the range bins mode, and moves the tapswitch 3l over the taps l to 10 until maximum ampiitude Doppler signalis heard. The tap on which switch 31. is located is then noted and thecorresponding range is read off. By using the two modes of operation inthis manner the position of a target can be determined quickly.

The above-described correlation system whether operating in the rangebins mode or in the all range mode requires no elaborate fiitering, suchas the use of comb filters, and supplies a substantially noise freeDoppler signal to the ear phones. The reason for this is that when therepetitive code directly phase modulates the carrier wave to shift itsphase 180 degrees as described, the frequency spectrum of the resultingsignal consists of frequency components: zero plus target Dopplerfrequency, code repetition rate plus and minus Doppler frequency, twotimes code repetition rate plus and minus Doppier frequency, etc.Therefore, this resulting signal consists of frequency components spacedapart by a frequency equal to the code repetition rate minus twice thetarget Doppler frequency (which in this example is 6000 per second).Thus, in the present example the demodulated carrier applied to thevideo amplifier has a yfrequency spectrum having frequency components atthe target Doppler frequency, at 6000 cycles per second plus and minusthe Doppler frequency, at 1200 cycles per second plus and minus theDoppler frequency, etc. in the present example, the highest freq encyDoppler signal to be passed to the ear phones is one of 1500 cycles persecond. Therefore, by designing the audio ampiifier (or by inserting aseparate filter) to pass signal from about 30 cycles per second toslightly above 1500 cycles per second, as in this specific example, andto exclude signals in the region of 4500 cycles (6000-1500 cycles) andabove, the noise components due to uncorrelated signals, andparticularly those due to the code repetition rate, are eliminated.

From the foregoing it will be apparent that not only should the coderepetition frequency be higher than the highest Doppler frequency to heutilized, but preferably it should be at least two times said highestDoppler frequency to avoid overlap in the frequency spectrum of theDoppler frequency and the code repetition frequency (6000 in thisexample) minus the Doppler frequency.

Refer new to the balanced circuit of FIG. 2 which caneels out anyamplitude modulation if such modulation is introduced by the phasemodulator 11. A small portion of the transmitted signal is taken off theline 16 by a coupier 56 and fed to a radio frequency attenuator 57 whichis adjusted to supply signal of the proper strength to the diode 1%. Thediode 19h is connected in polarity opposition to the mixer diode 13a sothat the signal appearing across its load resistor S8 is of oppositepolarity to that appearing across the load resistor 51 of diode 19a.

assasas 7 The resistors 51 and 58 may have a resistance of 220 ohmseach, for example.

The signal appearing across load resistor 58 is applied to the upper endof the sum resistor 52. Thus, it tends to cancel the correspondingopposite polarity signal applied to the lower end of sum resistor 52.The resistor 52 may have a resistance of 500 ohms, for example.

Referring more specifically to the cancellation operation, and takingcertain values by way of example, assume the input power to thecirculator by way of conductor 13 is 6 dbm. (Note that zero dbm=lmilliwatt.) The circular leakage attenuation is about -18 dbm, so thatthe detected amplitude modulation at the load resistor 51 is -12 dbm.

The transmitted signal picked up by the coupler 56 is attenuated about+14 dbm by the coupler and the RF attenuator 57, so that the detectedamplitude modulation at the load resistor 58 is -8 dbm. Thus, thevoltage of the detected amplitude modulation is greater at the top ofsum resistor 52 than at the bottom, resulting in an amplitude modulationbalancing out position for tap S3 toward the bottom of the resistor 52.This is desired so that the Doppler signal output of mixer diode 19awill pass through a minimum amount of the sum resistor and thus suferminimum attenuation.

The Doppler signal output of diode 19a is not balanced out by theDoppler signal output of diode 19b because the reflected signal thatreaches diode 19b is greatly attenuated (-1l'r dbm in this example),Whereas the reflected signal reaching diode 19a through the circulator iis not attenuated.

It may be noted that the input end of the video amplitier provides aload impedance (of about 1000 ohms in this example) into which thesignal from tap S3 feeds. A capacitor 61 connected between the tap 53and ground functions as an envelope detection lter cooperating with theamplitude detection function of the diodes 19a and 19b.

It will be evident that any amplitude modulation that may be present maybe balanced out by adjusting the tap 53 to the proper position.

The specific example of a suitable phase modulator (modulator l11)previously referred to will now be described.

Refer to FIG. 5.

The phase modulator comprises a three port circulator 7S, a shorttransmission line 76 leading from the second port, and a varactor diode77 that, together with an inductive impedance described later,terminates the transmission line 76. The 9250 mc. input signal from theoscilt lator 10 (FIG. 1) is applied to the first port through atransmission line 74 in this case a coaxial line.

It is a characteristic of a circulator that improper termination of aport causes reection of part of the signal into the next port (in thedirection of the arrow on the circulator 75). If the termination isreactive, which is the case here where the termination is eitherinductive or capacitive depending upon the varactor capacity, the energyis completely reflected from the second port to the third port and outthe line 13. Also, the change in phase of the reflected signal is afunction of the change in the capacity of the terminating varactor 77.

The varactor diode 77 is back-biased su'iciently by a bias voltagesource (not shown) so that when the code signal is applied the varactoris backed-biased during both the +1 Value and the -1 value of the codeand, therefore, has a certain capacitive value for each code value. Thevaractor has one capacity at the +1 code value resulting in a phaseshift of one value for the retiected carrier, and it has a differentcapacity at the -l code value resulting in a phase shift of a differentvalue for the reflected carrier. These two phase shift values are madeto diler by 180 degrees.

In the example being described, the transmission lines 74, 13 and 76 arecoaxial lines having a characteristic impedance of fty ohms. Thecirculator 75 also has a characteristic impedance of fifty ohms.

The transmission line 76 may be considered to be terminated `at thejunction point 78 where varactor 77 is connected to the center conductor79. The length of line 76 (from the circulator to the junction point 78)is not critical. ln the present example it is between 1% and 1Awavelength (a) long, but it may be l/zi or more in length. Thewavelength is the wavelength of the 9250 mc. carrier.

A coaxial line section 79 extends from the line 76 for the purpose ofadding inductance to the termination of line 76. The end of line 79 isshort circuited. The point of short circuiting is made adjustable by anadjustable directcurrent short circuit connection 81. The coaxial linelength from the junction point 78 to the short circuit 81 is less than1,/4 so that the impedance looking into the line section 7 9 fromjunction point 7 8 is inductive.

As described, one side of varactor 77 is connected to the centerconductor 79. The other side of the varactor is connected to the centerconductor 82 of a coaxial line 83 through which the code signal is fedto the varactor. The code is applied through a direct-current amplier(not shown), the proper back bias being applied to the varactor. It willbe noted that the code is applied to the varactor through the centerconductor 82; and through the outer conductor ot' line 33, the outerconductor of line 79, and through the short 81 and the center conductorof line 79 to the other side ofthe varactor.

An adjustable alternating-current shorting connection S4 is provided inthe line S3. This short is made A-C instead of D-C to avoid shorting outthe code signal. The short 84 is positioned so that the line 83 lookinginto it from the varactor is capacitive. The short 84 is adjusted to apoint where it effectively removes the inductance and stray capacityassociated with the varactor, i.e., the line capacity provided by theshort and the reactance associated with the varactor are made toresonate at 9250 From the foregoing it will be seen that the line 76 isterminated by both capacity and inductance, the capacity being providedby the varactor 77, and the inductance being provided by the shortedline section 79.

The above-described phase modulator may be adjusted to provide thedesired 180 degree phase shift as follows. The code signal with the backbias is adjusted so that at one code value (the +1 code value, forexample) the back bias voltage has a selected maximum value so that thevaractor has a selected minimum capacitance value, and so that at theother code Value 1 in this example) the varactor has a selected maximumcapacitance value. This change in varactor capacity may be in the orderof a two to one or a three to one change.

The code signal values corresponding to +1 and -1 are now supplied tothe line 83 by manual switching. First the +1 value signal is applied sothat the varactor has the selected minimum capacity value. Under thiscondition, looking into the line 76 from the circulator, the line 76looks predominately inductive, and the line 76 appears to have a lengthL0.

Next the -1 value signal is applied so that the varactor has theselected maximum capacity value. Now the line 76 looks predominatelycapacitive looking into it from the circulator, and the line 76 appearsto have a length L1, the length L1 being greater than the length L0. Thedifference between the lengths 1 4, and L1 is adjusted until this lengthdifference is exactly Mm, thus, providing a 180 degree phase shift sincethe carrier is reflected, i.e., it passes down the line 76 and back tothe second port. Therefore, in this round trip the carrier passesthrough a length of line that is V2A greater when the line length is L1than when it is L0.

This desired difference in the line lengths L1 and L0 is obtained byadjusting the position of the shorting bar 81. The correct position ofthe short S1 is determined by means of a phase meter connected to theinput line 74 and the output line 13. When the phase difference betweenthe input signal and the output signal is shown to be 180 degrees, theadjustment is complete.

In the course of adjusting the D-C short 81 to the correct position, theA-C short 84 usually should also be adjusted. It is known that the A-Cshort S4 is at its proper position when the D-C short 81 becomeseffective so that its adjustment results in the desired 180 degree phaseshift- Phase modulation by the above-described phase modulator produceslittle or no amplitude modulation. This is because the 180 degree phaseshift is obtained by a varactor capacitor change (such as two or threeto one) that is small enough so that the varactor is substantiallyentirely capacitive in operation, any lossy resistive effects beingnegligible. It may be noted also that the coaxial lines 76, 79 and 83have a low internal loss.

What is claimed is:

1. A correlation system in which a Wave is transmitted to a target andreected back which comprises means for generating a code in the form ofa square wave which has a quarter wave period approximately equal to thetransit time to a target and return where the target is at maximumrange,

means for generating a carrier wave,

means including a phase modulator to which a modulating signalcomprising said code is applied for reversing the phase of said carrierwave with each change in code amplitude,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after reflection from said target,said received Ieected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said retlected signal and areference signal consisting of an attentuated portion of said phasemodulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receivermixer output containing Doppler signal,

means for generating a comparison signal in the form of a square waverecurring at twice the frequency of said square wave code and insynchronism therewith,

means for applying said comparison signal to sai-:l correlator whereby aDoppler signal representative of any moving target within said maximumrange apr Ipears in the output signal of said correlator, and

a utilization device to which the Doppler signal from said correlator issupplied.

2. A correlation system which comprises means for generating arepetitive code in the form of a square wave which has a certainrepetition rate,

means for generating a carrier wave,

means including a phase modulator to which a modulating signalcomprising said code is applied for reversing the phase of said carrierwave with each change in code amplitude,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after retlection from said target,said received reected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reected signa-l and areference signal consisting of an attentuated portion of said phasemodulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receivermixer output containing Doppler signal,

means for generating a comparison signal in the form of a square Wavehaving a repetition rate that is twice that of the square wave formingsaid code,

means for applying said comparison signal to said correlator whereby aDoppler signal representative of a moving target appears in the outputsignal of said correlator when the moving target is within the maximumrange of said system where the transit time to and from a target atmaximum range is equal to onefourth the period of said square wave code.

a utilization device, and means for supplying to said device only thatportion of said correlator output which has a frequency below a certainfrequency limit,

said certain repetition rate of said code being at least twice thefrequency of said frequency limit.

3. A correlation system having two modes of operation,

which comprises code generating means for generating a pseudo-randomcode which varies in amplitude and which has a certain repetition rate,

said code generating means including a clock and a shift register drivenby said clock,

a second code generating means for generating a repetitive code in theform of a square wave which has a repetition rate greater than saidcertain repetition rate,

said second code generating means including a frequency divider drivenby said clock,

means for generating a correlation comparison signal in the form of asquare wave having a repetitive rate twice that of said square wavecode,

said last means including a frequency divider driven by said clock,

means for generating a carrier wave,

means including a phase modulator to which a modulating signalcomprising a selected one or" said codes is applied for reversing thephase of said carrier wave with each change in code amplitude,

said square Wave code being selected for the first mode of operation,and said pseudo-random code being selected for the other mode ofoperation,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after reflection from said target,said received reliected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reilected signal and areference signal consisting of an attenuated portion of said phasemodulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receivermixer output containing Doppler signal,

means for delaying said pseudo-random code a selected amount to producea delayed code,

means for multiplying said delayed code by said pseudorandom codeundelayed to obtain a resulting product signal,

means for applying said correlation comparison signal to said correlatorwhen said system is operated in said rst mode,

means for applying said product signal to said correlator when saidsystem is operated in the other mode, whereby when operated in saidother mode a Doppler signal representative of a moving target appears inthe output of said correlator when said selected code delay issubstantially equal to the transit time of the received reected signal,and whereby when operated in said tir-st mode a Doppler signalrepresentative of a moving target appears in the output of saidcorrelator when the moving target is within the maximum range of Saidsystem where the transit time to and from a target at maximum range isequal to oneforth the period of said square Wave code,

a utilization device, and means for supplying to said device only thatportion of said correlator output which has a frequency below a certainfrequency limit,

said certain repetition rate of said code being at least twice thefrequency of said frequency limit.

4. A correlation system having an all range mode of operation and arange bins mode of operation, which comprises means for generating anall range code which varies in amplitude,

means for generating a range bins code which varies in amplitude,

means for generating a carrier wave,

means including a phase modulator to which a modulating signalcomprising a selected one of Said codes is applied for reversing thephase of said carrier wave with each change in code amplitude,

said all range code being selected for the all range mode of operation,and said range bins code being selected for the range bins mode ofoperation,

means for transmitting said phase modulated carrier toward a target,

means for receiving said carrier wave after reection from said target,said received reiiected signal being delayed by the transit time,

a receiver mixer,

means for supplying to said receiver mixer said reflected signal and areference signal consisting of an attenuated portion of said phasemodulated carrier prior to transmission,

a correlator,

means for supplying to said correlator the component of said receivermixer output containing Doppler signa-l,

means for generating an all range correlation comparison signal,

means for delaying said range bins code a selected amount to produce adelayed code,

means for multiplying said delayed code by said range bins codeundelayed to obtain a range bins correlation comparison signal,

means for applying said all range correlation comparison signal to saidcorrelator when said system is operated in the all range mode,

said all range code being such that in the all range mode there iscomplete correlation of the Doppler component from the receiver mixerand the all range comparison signa-l when the retiected signal is from atarget at maximum range resulting in maximum Doppler output from thecorrelator, and only partial correlation When the reliected signal isfrom a target at less than maximum range resulting in less than maX-imum Doppler output from the correlator,

means for applying said range bins correlation comparison signal to saidcorrelator when said system is operated in the range bins mode,

said range bins code being such that in the range bins mode there iscomplete correlation of the Doppler component from the receiver mixerand said range bins correlation comparison signal at any one of aplurality of target ranges at each of which said selected code delay issubstantially equal to the transit time of the received reflected signalresulting in maximum Doppler output from the correlator,

and a utilization device to which is supplied at least a portion of saidcorrelator Doppler output,

said system including means for switching from one mode of operation tothe other mode of operation.

No references cited.

RODNEY D. BENNETT, Primary Examiner.

il. P. MORRIS, Assistant Examiner.

